This is a first-in-human, open-label, multi-center study. The study is composed of the following periods: - Screening - Treatment - Follow-up (mandatory safety follow-up and, whenever applicable, efficacy follow-up visits) - Long-term follow-up. The thorium-227 dose escalation will be carried out in a step-wise fashion, using two antibody doses, 10 and 30 mg, which will proceed independently for each dose of thorium-227. Enrollment of patients in the 10 mg and 30 mg cohorts will be allocated according to a pre-defined cohort management plan agreed between the sponsor and investigators and the decision to escalate to the next thorium-227 level will be taken independently for each cohort. No intra-subject dose escalation will be allowed. At any time point of the dose escalation, testing of one of the two doses may be interrupted on the basis of emerging clinical data and/or substituted by testing of a different antibody dose within the range of 10-50 mg, after discussion and agreement between the sponsor and the investigators.